1. Field of the Invention
This invention relates to a multi-layer extruded article having good impermeability to gases and good adhesion between layers which comprises a gas-barrier layer and a polyolefin layer, the gas-barrier layer being formed of a saponification product of an ethylene-vinyl acetate copolymer.
2. Description of the Prior Art
Polyolefins, owing to their excellences in impermeability to water vapor, resistance to chemicals, transparency, hygiene, etc., have heretofore found extensive utility in applications to packaging materials. Nevertheless they are not completely satisfactory in the sense that they exhibit high degrees of permeability to oxygen gas, carbon dioxide gas and other similar gases and, accordingly, packages made thereof fail to preserve their contents intact for a long time.
In contrast, a saponification product of an ethylene-vinyl acetate copolymer having a low ethylene content (on the order of not more than 60 mol percent, for example) and a high saponification degree (on the order of not less than 80 percent, for example) (hereinafter generally called "saponification product") has an advantage that it exhibits notably lower permeability to oxygen gas than polyolefins. On the other hand, this saponification product suffers from a disadvantage that, because of its hydrophilicity, it exhibits high permeability to water vapor and, upon exposure to a highly moist atmosphere, shows enhanced permeability to gases such as oxygen gas.
In the circumstances, there has been felt a need of developing, as a packaging material combining the characteristics of polyolefins and those of the saponification product mentioned respectively above, a multi-layer extruded article formed in at least two layers, namely, comprising one layer of the saponification product and one polyolefin layer laminated on at least one of the opposite sides of the saponification product layer, particularly a multi-layer container formed in three or more layers, namely, comprising one layer of the saponification product and polyolefin layers laminated on the opposite sides of the saponification product layer.
When the multi-layer container mentioned above is produced by simultaneously extruding one layer of the saponification product (hereinafter called "gas-barrier layer") and at least two polyolefin layers in accordance with the co-extrusion process, because the saponification product and the polyolefin possess no affinity for each other, the extruded layers adhere to each other with insufficient fastness and, therefore, readily separate from each other. In most cases, accordingly, a multi-layer containers produced by this method fail to withstand the conditions of their actual use.
As a solution of this problem, Japanese Patent Publication No. 48512/1976 has proposed a method which comprises the addition to the saponification product of some other polymer possessed of high affinity for both the saponification product and the polyolefin. Specifically disclosed by this Japanese Patent publication is a laminated article which comprises a gas-barrier layer and polyolefin layers, the gas-barrier layer being made of a first saponification product of an ethylene-vinyl acetate copolymer having an ethylene content of 10 to 65 mol percent and a saponification degree of not less than 80 percent and a different kind of polymer. As the different kind of polymer, there is used either a second saponification product of an ethylene-vinyl acetate copolymer having an ethylene content of 70 to 98 mol percent and a saponification degree of not less than 20 percent or a mixture of the second saponification product with a polyolefin. The aforementioned addition to the first saponification product of either the second saponification product or the mixture thereof with a polyolefin indeed serves the purpose of improving the fastness with which the gas-barrier layer and the polyolefin layers are allowed to adhere to each other, but it nevertheless entails a problem of causing a degradation in the outstanding impermeability to gases (gas-barrier property) which constitutes a salient characteristic of the first saponification product used as the principal component of the gas-barrier layer. With respect to the permeability to oxygen gas, for example, it is shown in the aforementioned Japanese patent publication that the value which is not more than 5 cc/m.sup.2 /24 hours where the gas-barrier layer is made solely of the first saponification product is degraded to the order of 15 to 30 cc/m.sup.2 /24 hours where the gas-barrier layer is made of the composition of the first saponification product and the second saponification product combined, or not combined, with the polyolefin.